diff --git a/packages/smithy-http/src/smithy_http/aio/crt.py b/packages/smithy-http/src/smithy_http/aio/crt.py
index 88531e95a..58fa3ebae 100644
--- a/packages/smithy-http/src/smithy_http/aio/crt.py
+++ b/packages/smithy-http/src/smithy_http/aio/crt.py
@@ -292,8 +292,14 @@ async def _marshal_request(
         """Create :py:class:`awscrt.http.HttpRequest` from
         :py:class:`smithy_http.aio.HTTPRequest`"""
         headers_list = []
+        if "Host" not in request.fields:
+            request.fields.set_field(
+                Field(name="Host", values=[request.destination.host])
+            )
+
         for fld in request.fields.entries.values():
-            if fld.kind != FieldPosition.HEADER:
+            # TODO: Use literal values for "header"/"trailer".
+            if fld.kind.value != FieldPosition.HEADER.value:
                 continue
             for val in fld.values:
                 headers_list.append((fld.name, val))
diff --git a/packages/smithy-http/src/smithy_http/interfaces/__init__.py b/packages/smithy-http/src/smithy_http/interfaces/__init__.py
index 0c3ac3cc0..2d1445199 100644
--- a/packages/smithy-http/src/smithy_http/interfaces/__init__.py
+++ b/packages/smithy-http/src/smithy_http/interfaces/__init__.py
@@ -98,6 +98,10 @@ def __len__(self) -> int:
         """Get total number of Field entries."""
         ...
 
+    def __contains__(self, key: str) -> bool:
+        """Allows in/not in checks on Field entries."""
+        ...
+
     def get_by_type(self, kind: FieldPosition) -> list[Field]:
         """Helper function for retrieving specific types of fields.
 
diff --git a/packages/smithy-http/tests/unit/aio/test_crt.py b/packages/smithy-http/tests/unit/aio/test_crt.py
index 3ca5197cc..d5c695417 100644
--- a/packages/smithy-http/tests/unit/aio/test_crt.py
+++ b/packages/smithy-http/tests/unit/aio/test_crt.py
@@ -1,9 +1,13 @@
 #  Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 #  SPDX-License-Identifier: Apache-2.0
 from copy import deepcopy
+from io import BytesIO
 
 import pytest
 
+from smithy_core import URI
+from smithy_http import Fields
+from smithy_http.aio import HTTPRequest
 from smithy_http.aio.crt import AWSCRTHTTPClient, BufferableByteStream
 
 
@@ -12,6 +16,22 @@ def test_deepcopy_client() -> None:
     deepcopy(client)
 
 
+async def test_client_marshal_request() -> None:
+    client = AWSCRTHTTPClient()
+    request = HTTPRequest(
+        method="GET",
+        destination=URI(
+            host="example.com", path="/path", query="key1=value1&key2=value2"
+        ),
+        body=BytesIO(),
+        fields=Fields(),
+    )
+    crt_request = await client._marshal_request(request)  # type: ignore
+    assert crt_request.headers.get("host") == "example.com"  # type: ignore
+    assert crt_request.method == "GET"  # type: ignore
+    assert crt_request.path == "/path?key1=value1&key2=value2"  # type: ignore
+
+
 def test_stream_write() -> None:
     stream = BufferableByteStream()
     stream.write(b"foo")